1. Field of the Invention
The present invention relates to an image sensing apparatus and a control method therefor, and more particularly, to an image sensing apparatus that carries out dark shading correction and a control method therefor.
2. Description of the Related Art
Conventionally, a variety of different dark shading correction methods have been proposed for correcting dark current noise in image sensors such as CCD sensors, CMOS sensors, and the like used in digital camera and other image sensing apparatuses.
FIG. 6 is a schematic block diagram showing an example of a functional configuration of a digital cameras for carrying out conventional dark shading correction. In the camera shown in FIG. 6, a subject is photographed by an image sensor 1001 under shooting conditions (aperture, shutter speed, sensitivity) set by exposure control operations, and accumulated electrical charge signals are read out from the image sensor 1001. The electrical charge signals are converted into digital signals by an A/D converter 1002 and recorded in a buffer memory 1003 as an image signal (hereinafter “subject image signal”). Then, under the same shooting conditions, shooting is carried out with the image sensor 1001 shielded from light, and the electrical charge signals thus acquired are converted into digital signals by the A/D converter 1002. An image signal acquired in this manner is hereinafter referred to as a black image signal.
At the same time as the black image signal is output from the A/D converter 1002, the subject image signal accumulated in the buffer memory 1003 is read out. Then, by subtracting the black image signal from the subject image signal at a subtraction circuit 1004, dark shading correction is carried out. An arbitrary offset value (for example, a 10-bit signal of approximately 32 LSB) is added to the dark shading-corrected subject image signal at an offset circuit 1005. The subject image signal to which an offset value has been added is then sent to a signal processing circuit 1006 and an image signal for one image is generated. The image signals generated in this manner are accumulated in a general purpose storage medium 1007 such as an SD card or the like.
However, with the dark shading correction method described above, because it uses two image signals (the subject image signal and the black image signal), random noise increases by a factor of √2 compared to a case in which dark shading correction is not carried out. In addition, because shooting operations and readout are carried out for two images, it takes time for shooting to finish.
In an invention described in Japanese Patent Laid-Open No. 11-289494, it is proposed that the black image signal be divided into a plurality of blocks, signals integrated at each block, and the integrated signals enlarged to suit the resolution of a subject image and then subtracted from the subject image. According to this method, because the random noise of the black image signal is reduced, degradation due to noise in the image signal after dark shading correction can be virtually eliminated.
However, with the method described in Japanese Patent Laid-Open No. 11-289494 also, in order to obtain an image signal of a single subject image, the image signal shooting operation and the readout operation are executed twice, as with the method described above with reference to FIG. 6. As a result, there is the problem that it takes time for shooting to finish, and during continuous shooting there is a considerable drop-off in shooting speed.